Debugging of embedded solutions has always been a difficult job. As processors become faster and more complex, debugging and development with the current debug technology becomes more difficult. In order to address these complex issues, greater visibility into the program operation is needed. Three areas in which greater visibility is desired are program counter tracing, cycle accurate profiling, and load and store data logging. Access to this data may be available through a dedicated Debug Port. However, each of these problems demands a tremendous amount of information. Simply supplying a large number of high frequency pins to view all of this data is neither practical nor cost effective, and an encoding scheme is needed to further compress all of this data. An encoding has been used that encodes Program Counter (PC) tracing, cycle accurate timing of all instructions, and load and store data logging. All of this data can be transmitted across the same pins on the Debug Port.
The debug port is a tool that provides for the export of software or hardware generated trace information to an external recorder. The trace port utilizes a transmission format that addresses the requirements without noticeably compromising the format efficiency for any given implementation. The transmission format primitives are viewed as a trace export instruction set. All processors use this instruction set to describe the system activity within a device. Each processor can describe the system activity in any manner that uses the instruction set and the rule set governing its use.
It is important to note that the external transmission rates/pins are fixed by the deployed receiver technology. These rates will remain relatively constant over time. This implies that as CPU clock rates increase, there will be increasing pressure to optimize the format to get the most compressed representation of system activity. This will be necessary just to maintain the status quo. Fortunately, the transmission format used provides an efficient means to represent the system activity. However, this efficiency comes at the expense of a larger on-chip hardware expenditure in order to gain the compression efficiency. This gives the processors the capability to improve the efficiency of their export bandwidth as it is stressed by CPU clock rate increases. The steady march to faster CPU clock rates and denser manufacturing processes will necessitate taking advantage of all compression opportunities and the best available physical transmission technology.
The transmission format is designed to provide designers the ability to:
Optimize bandwidth utilization (most real information sent in minimum bits/second);
Chose less efficient but more cost effective representations of system activity.
Mix of both of the above approaches (i.e. optimize PC trace transmission efficiency while implementing less efficient memory access export).
This gives different processors the ability to represent their system activity in forms most suitable to their architecture.
Tradeoffs have to be made since there are numerous cost/capability/bandwidth configuration requirements. Adjustments can be made to optimize and improve the format over time.
The transmission format remains constant over all processors while the nature of the physical transmission layer can be altered. These alterations can take three forms:
Transmission type (differential serial or conventional single ended I/O);
Number of pins allocated to the transmission;
Frequency of the data transmission.
This means that the format representing the system activity can and is viewed as data by the actual physical mechanism to be transmitted. The collection and formatting sections of the debug port should be implemented without regard to the physical transmission layer. This allows the physical layer to be optimized to the available pins and transmission bandwidth type without changing the underlying physical implementation. The receiver components are designed to be both physical layer and format independent. This allows the entire transmit portion to evolve over time.
A 10-bit encoding is used to represent the PC trace, data log, and timing information. The trace format width has been decoupled from number of transmission pins. This format can be used with any number of transmission pins. The PC trace, Memory Reference information, and the timing information are transmitted across the same pins.
Packets can contain opcodes or data, or both. A code packet contains an opcode that indicates the type of information being sent. The opcode can be 2 to 10 bits long. The remainder of the code packet will hold data associated with that opcode.
In many cases, additional data needs to be associated with an opcode. This data is encoded in subsequent packets referred to as data packets. Data packets contain information that should be associated with the previous opcode.
A sequence of packets that begins with code packet and includes all of the data packets that immediately follow the code packet is referred to as a command. A command can have zero or more parameters. Each parameter is an independent piece of data associated with the opcode in the command. The number of parameters expected depends on the opcode. The first parameter of a command is simply encoded using data packets following a code packet. The first data packet of subsequent parameters is marked with the 10 opcode.
The interpretation of a command is dependent on two factors, the opcode of the command, and the number of parameters included in the command. In other words, a code packet has one meaning if it is immediately followed by another code packet, but the same packet can take on an entirely different meaning if it is succeeded with data packets. Trace opcodes are shown in Table 1.
TABLE 1 00000 0000No Information/End of Buffer000000 0001Start Repeat Single000000 0010PC Trace Gap000000 0011Register Repeat000000 0100NOP SP loop000000 0101SPLOOP marker000000 0110Timing Trace Gap000000 0111Command Escape000000 1000Exception Occurred000000 1001Exception Occurred with RepeatSingle000000 1010Block Repeat 0000000 1011Block Repeat 0 with Repeat Single000000 1100Block Repeat 1000000 1101Block Repeat 1 with Repeat Single000000 1110Memory Reference Trace Gap000000 1111Periodic Data Sync Point000001 0xxxTiming Sync Point000001 1xxxMemory Reference Sync Point000010 xxxxPC Sync Point/First/Last/000011 000xPC Event Collision000011 001xReserved000011 01xxReserved000011 1xxxReserved00010x xxxxExtended Timing Data00011x xxxxCPU and ASIC Data0010xx xxxxReserved001100 0000Memory Reference Trace Gap (legacy001100 0001Periodic Data Sync Point (legacy0011xx xxxxMemory Reference Block01xxxx xxxxRelative Branch Command/RegisterBranch10xxxx xxxxContinue11xxxx xxxxTiming
The PC Sync Point command is used to indicate several program events. It is used to mark periodically generated PC and timing synchronization points, the start of a trace segment, the end of a trace segment, a debug halt, Reset a trigger and other events. A parameter in this command conveys the reason for the PC Sync Point.
The PC Sync Point, Timing Sync Point, and Data Sync Point commands are used together to align the PC trace data with the timing information. When PC Sync Points are generated, they initiate Memory Reference Sync Points. Timing Sync Points are generated only when timing is turned on. When timing is turned on, a Timing Sync Point and PC Sync Point pair serves as a starting point for interpretation of the trace data. When timing is off, a PC Sync Point is sufficient for interpretation of the trace data. When PC Trace is turned off, Data Sync Points are sufficient to interpret the trace.
Inserting periodic Sync Points in the trace stream gives the user multiple points at which to begin interpretation of the trace data. It also allows the interpretation of the data when the buffer in the trace receiver overflows and the Trace Start command or earlier Sync Points are discarded.